Raman study of theα−βcristobalite phase transition in AIPO4

The Raman spectra of small single crystals of the cristobalite form of aluminum phosphate (AIP${\mathrm{O}}_4$) have been obtained in both the $\alpha$ and $\beta$ phase. Particular emphasis has been placed on the behavior of the phonons near the phase-transition temperature. The transition is first order, but the latent heat and discontinuity in lattice parameters are small enough to permit cycling of the sample through the transition without damage. A soft mode is observed which has energy 102 ${\mathrm{cm}}^{-1\mathrm{}}$ at ambient temperatures. The observed spectra confirm the assignments of space groups $C222\mathrm{} \left(D_2^5\right)$ and $I\overline{4}m2\mathrm{} \left(D_{2d}^9\right)$ for the $\alpha$ and $\beta$ phases, respectively. Phonon comparisons are made with isomorphic BP${\mathrm{O}}_4$ and Si${\mathrm{O}}_2$.